1. Field of the Invention
The present invention relates to a low-iron-loss grain-oriented electromagnetic steel sheet and also to a method of producing such a steel sheet.
2. Description of Related Arts
Grain-oriented electromagnetic steel sheets are used mainly in transformer cores and, hence, are required to have superior magnetic characteristics. In particular, it is important that the steel sheet minimize energy loss, also known as iron loss, when used as the core material.
In order to cope with such a demand, various techniques have been proposed such as enhancing the degree of alignment of crystal texture in (110)[001] orientation, increasing electric resistivity of steel sheet by enriching the Si content, reducing the impurity content, reducing the sheet thickness, and so forth. Presently, steel sheets of 0.23 mm or thinner, having iron loss W.sub.17/50 (iron loss exhibited when alternatingly magnetized at 50 Hz under maximum magnetic flux density of 1.7 T) of 0.9 W/kg or less are successfully produced. However, the limits of iron loss reduction attainable through metallurgical techniques have likely been reached.
In recent years, therefore, various attempts and proposals have been made to artificially realize fine magnetic domains in steel sheets as a measure for achieving a remarkable reduction in the iron loss. One such attempt or proposal, actually carried out in industrial scale, involves irradiating the surface of a finish-annealed steel sheet with a laser beam. The steel sheet produced by this method possesses regions of high dislocation density, formed as a result of the high energy imparted by the laser beam. These regions of high dislocation density cause 180.degree. magnetic domains to be finely defined, thus contributing to reduction in iron loss.
It should be noted, however, that steel sheets thus produced cannot be used as wound transformer cores because the high temperatures associated with the required strain-relieving annealing increase iron loss by destroying the high dislocation density regions.
Methods have been proposed for enabling such strain-relieving annealing. For instance, Japanese Patent Publication No. 62-54873 discloses a method in which insulating coating on a finish-annealed steel sheet is locally removed by, for example, laser beam or mechanical means, followed by pickling of the local portions where the insulating coating has been removed. Japanese Patent Publication No. 62-54873 also discloses a method in which linear grooves are formed in the matrix iron by scribing with mechanical means such as a knife, and the grooves are filled by a treatment for forming a phosphate type tension imparting agent. Meanwhile, Japanese Patent Publication No. 62-53579 discloses a method in which grooves of 5 .mu.m or deeper are formed in finish-annealed steel sheet by application of a load of 90 to 220 kg/mm.sup.2, followed by heat treatment conducted at 750.degree. C. or above.
Japanese Patent Publication No. 3-69968 discloses a method in which a steel sheet which has undergone finish cold rolling is linearly and finely fluted in a direction substantially perpendicular to the direction of rolling.
In the known art described above, linear grooves or flutes are formed in the surface of the steel sheet, and the magnetic poles appearing near the grooves or flutes finely define magnetic domains. It is considered that such fine definition of magnetic domains is one of the reasons why the iron loss is reduced.
Thus, low-iron-loss steel sheets which can be subjected to strain-relieving annealing have become available by virtue of the methods described above. It has been found, however, that such steel sheets are sometimes significantly inferior to the steel sheets of the type disclosed in Japanese Patent Publication No. 57-2252 which have linear high dislocation density regions.